This invention generally relates to the technology of analyzing the nature of dust particles contained in the environmental air by observing light beams scattered from the dust particles.
Said analysis primarily consists in giving information on the nature of the atmospheric dust particles, namely, the various forms thereof such as fibrous, mineral, liquid, bacterial, etc., on the size of said dust particles and the frequency at which said dust particles appear in the air.
The principle of analyzing dust particles by light beams scattered therefrom is based on the fact that light beams scattered from dust particles essentially include information on the size, refractive index and shape of said dust particles. The object of this invention is to meet a strong demand for further improvement of the dust particle-analyzing technology by engineering for control of air pollution as well as for cleaning air present in the industrial zone and hospitals.
This invention relates particularly to the application of a laser as a light source for the observation of scattered light. Recent development in the ultraviolet and shorter visible wave gas laser has an important implication in rendering the new development involved in this invention available for practical use.
Three types of commercially available dust counters were described by W. R. Zinky (J. Air Pollution Control Assoc. 12, 578 (1962), J. M. Raudall and J. D. Keller (Am. Ind. Hyg. Assoc. J. 29, 257 (1968)) and D. Scinclair (J. Air Pollution Control Assoc. 17, 105 (1967)) respectively. These instrument differ from each other mostly in the geometry of illuminating and observing optical systems. The first one uses an observing optical system having its axis disposed perpendicular to that of the illuminating optical system. The second one contains an observing optical system receiving a forward hollow cone of a desired solid angle coaxial with that which is emitted from the illuminating optical system. All these three instruments are alike in applying white light as a source of illumination and in employing an observing optical system having a large solid angle.
According to the theory of light scattering by a spherical dielectric particle proposed by G. Von Mie (Annalev d. Physik, 25, 378 (1908)), the scattering cross-section S, that is, the scattered light intensity per unit solid angle of observation per unit intensity of the illuminating light shows a complicated dependence on the solid conical angle as a function of the refractive index n and the reduced dust particle diameter .delta. especially for larger values of 67 than 2. The reduced dust particle diameter .delta. is defined as: EQU .delta.=2.pi.a/.lambda. (1)
where 2a and .lambda. denote the diameter of the dust particle and the wavelength of the illuminating light respectively. The scattering cross-section S shows a rather smooth dependence on 2a and is relatively insensitive to n as averaged over the wavelength .lambda. of the white light source and over the solid angle of observation. Various light scattering dust counters proposed to date which use white light and contain an observing optical system for receiving a forward hollow cone of a large solid angle in view of the above-mentioned facts, as well as the aforesaid three instruments claim the good light-collecting efficiency of the observing optical system and the reliable deduction of the particle diameter 2a even when an operator lacks the knowledge of the refractive index n of the dust particle.
A light scattering apparatus using a He-Ne laser having a wavelength of 633 nm as an illuminating source and using a forward hollow cone of a slim solid angle for observation of dust particles was proposed by W. Kaye (analytical Chemistry 45, 221A, 1973).
When used for observation of the Debye-Zimm scattering from a liquid solution and Rayleigh scattering from dust particles dispersed in a liquid carrier, the apparatus proposed by W. Kaye demonstrated the favourable nature of a laser used as a light source for a aerosol light-scattering apparatus. The highly coherent nature of a laser attains a high luminosity in a light scattering space even by use of an illuminating optical system emitting a beam of light having an acute conical solid angle.
Further, the above-mentioned nature of a laser enables a sufficient amount of scattered light energy to be collected even in the slim solid angle region of a forward hollow cone received by the observing optical system, thereby attaining the observation of the substantially true forward light scattering just outside the cone emitted from the illuminating optical system without any harmful instrumental background light scattering which might occur in the absence of dust particles.
According to Mie's theory referred to above, the light-scattering cross-section S of a spherical dielectric particle for a small .delta. is given as: EQU S =(.lambda./2.pi.).sup.2 (n.sup.2 -1).sup.2 (n.sup.2 +2).sup.-2 .delta.6 (2)
Equation (2) shows that the light-scattering cross-section S is proportional to .lambda..sup.-4 and (2a ).sup.6 for a small .delta.. Accordingly, it is essential to use a light source having a shorter wavelength for the detection of dust particles of small diameters. This partly explains the reason why the prior dust counters have failed to obtain good results with respect to dust particles having a finer diameter than 0.3 micrometer.